Substrate mounting for organic, dielectric, optical film

ABSTRACT

A method for optically coupling a thermoplastic material to an outer surface layer of an organic, dielectric, optical film and the resulting optical filter. Initially, a dielectric film is selected that includes (i) repeating optical layers of at least two polymers having different refractive indexes from each other, (ii) an exterior film surface, (iii) a refractive boundary along the exterior film surface, and (iv) a delamination threshold based on total thermal energy delivered to the film. A thermoplastic material which is miscible with the exterior film surface is fused to the refractive boundary with thermal energy below the delamination threshold to form a polydisperse region having a higher optical transmission than the refractive boundary. Add-on filters in the form of hardcoat layers, anti-reflection layers, holograms, metal dielectric stacks and combinations of these may be combined with the thermoplastic-film construct.

RELATED APPLICATION INFORMATION

[0001] This application is a Divisional application of U.S. patentapplication Ser. No. 09/955,903 filed on Sep. 19, 2001, pending.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to methods for mounting thermoplasticsubstrates to organic, dielectric films and the resulting opticalfilters. More particularly, it relates to methodologies for fusing,through a variety of industrial processes, thermoplastic substrates todielectric films without delamination while improving opticaltransmission at the film's refractive boundary.

[0004] 2. Description of the Prior Art

[0005] Dielectric stacks have previously been constructed by sputteringlayers of inorganic (metal) oxides onto polymeric substrates. The layersalternate between high and low indices of refraction, whereby incominglight encounters a partial mirror at each new layer. By creatingso-called quarter wave stacks, certain wavelengths can be destructivelyinterfered via Bragg diffraction. These constructs possess severaladvantages as follows: they are intimately, optically coupled to thesubstrate thereby providing high transmissions; they are customengineered for the shape and curvature of each particular part. However,numerous disadvantages exist with these constructs as follows: thelabor, materials and equipment costs for manufacturing are prohibitive;the mechanical coupling of metal oxide layers onto polymeric substratesis subject to delamination from flexural stress; the differing rates ofthermal expansion between the metal oxides and the polymeric substratecan also cause delamination.

[0006] Recently, dielectric stacks have been manufactured fromalternating layers of polymers having high and low indices ofrefraction, creating organic, optical, dielectric films. Severalexamples of these films are described in U.S. Pat. No. 5,882,774,entitled Optical Film, the contents of which are incorporated herein byreference thereto. The different polymers have similar mechanicalproperties and melting temperature profiles allowing hundreds of layersto be stacked and stretched to control the thickness and optically tunethe film. While this allows the film to be economically manufactured, aproblem exists in optically coupling the film to other components, forexample, an optical thermoplastic substrate. Because the delicate filmis highly susceptible to delamination, melting, burning or having itsdesirable transmission properties altered, previous attempts to mountthe film have been limited to adhesion via optical fluids or opticaladhesives. While these methods are adequate, they severely reduce thefilm's high transmission characteristics, by providing refractiveboundaries from film-to-adhesive and from adhesive-to-substrate.

[0007] Accordingly, it would be desirable to mount the film onto asubstrate with structural and mechanical integrity while maintaining, orenhancing, the film's transmission characteristics.

SUMMARY OF THE INVENTION

[0008] It is therefore an object of the invention to provide a varietyof methods for manufacturing optical filters that incorporate organic,dielectric films.

[0009] It is a further object of the invention to provide industrialprocesses for fusing thermoplastic substrates to dielectric filmswithout delamination while improving optical transmission at the film'srefractive boundary.

[0010] It is yet another object of the present invention to describecriteria for selecting chemical and material properties ofthermoplastics which can be effectively utilized in the film fusingmethods.

[0011] It is another object of the present invention to describecriteria for thermally fusing the film to a thermoplastic substratebased on the chosen manufacturing method.

[0012] It is a further object of the present invention to describe theoptical properties of the thermally fused region.

[0013] These and other related objects are achieved according to theinvention by a method for optically coupling a thermoplastic material toan outer surface layer of an organic, dielectric, optical film. Adielectric film is selected that includes (i) repeating optical layersof at least two polymers having different refractive indexes from eachother, (ii) an exterior film surface, (iii) a refractive boundary alongthe exterior film surface, and (iv) a delamination threshold based ontotal thermal energy delivered to the film. Next, a thermoplasticmaterial which is miscible with the exterior film surface is selected.The thermoplastic material is fused to the refractive boundary withthermal energy below the delamination threshold to form a polydisperseregion having a higher optical transmission than the refractiveboundary. The molecular weight of the thermoplastic is selected so thatthe melting temperature range of the thermoplastic overlaps the meltingtemperature range of the exterior film surface.

[0014] Fusing may be accomplished by film insert molding, where thethermoplastic is simultaneously molded into a substrate and fused to thefilm's refractive boundary. The molten resin is held above the resin'sglass transition temperature in the barrel and experiences a temperaturedrop as it enters the cavity to below the film's thermal delaminationthreshold.

[0015] Fusing may also be accomplished via laser welding where aradiation absorbing material is placed near one of the refractiveboundaries and irradiating the absorbing material through thethermoplastic or the film.

[0016] Fusing may further be accomplished by extruding the thermoplasticinto a substrate and bringing the film into contact with the substrateas the substrate's temperature drops below the thermal delaminationthreshold.

[0017] The optical filters manufactured according to the methods of theinvention, may have incorporated therein, organic absorber dyes, such asa UV absorbing dye, a visible light absorbing dye, a cosmetic dye, alaser absorbing dye, a near infrared absorbing dye, and infraredabsorbing dye and combinations thereof. Add-on filters in the form ofhardcoat layers, anti-reflection layers, holograms, metal dielectricstacks and combinations of these may be combined with thethermoplastic-film construct.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In the drawings wherein like reference numeral denote similarcomponents throughout the views:

[0019]FIG. 1 is a cross-sectional view of an organic, optical,dielectric film layered on a thermoplastic substrate with air andadhesive disposed therebetween;

[0020]FIG. 2 is a cross-sectional view of an organic, optical,dielectric film fused to a thermoplastic substrate pursuant to themethod, and resulting product, according to the invention;

[0021]FIG. 3A is an exploded cross-sectional view of a lens formedaccording to the invention provided with a hardcoat layer and ananti-reflective layer;

[0022]FIG. 3B is an exploded cross-sectional view of a lens formedaccording to the invention provided with a hologram and a protectivecap;

[0023]FIG. 3C is an exploded cross-sectional view of a lens formedaccording to the invention provided with an inorganic dielectric stackand a protective cap; and

[0024]FIG. 3D is an exploded cross-sectional view of a lens formedaccording to the invention provided with a hologram, an inorganicdielectric stack and a protective cap.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0025] In U.S. Pat. No. 5,882,774 films are stacked together to improvetransmission spectra. For example, a pair of 204-layer polarizers madefrom alternating layers of PEN and coPEN are laminated together. Morespecifically, the outer coPEN layer on one polarizer is laminated to theouter coPEN layer of the other polarizer. In this instance, an opticaladhesive is used: wherein the only important property of the adhesive isthat it possesses a refractive index as close to the coPEN as possible.In other instances a relatively thick skin layer is formed on theexterior surface of the film. In the above example, the skin layer wouldbe formed from PEN or coPEN. In any event, the skin layer is made fromone of the polymers used in the film.

[0026] As can be seen in FIG. 1, a problem exists in layering a film 20onto a substrate 40. For example, coupling a film 20 made fromalternating layers 22, 24 of PEN and coPEN (or PEN and PMMA) to asubstrate 40 having different mechanical, chemical or opticalproperties. The numbers 22 and 24 may also represent any pair ofpolymers used in organic, optical, dielectric films. In layering thefilm onto the substrate, an intermediate medium 30 occupies any spacesor gaps therebetween. The intermediate medium 30 may be adjacent one ofthe alternating layers 22 or 24, or it may be adjacent a skin 26 madefrom one of the polymers of the film and made several times thicker thanthe layers, for example. In the simplest case, the intermediate medium30 consists of air 32. Alternatively, the intermediate medium 30consists of an adhesive 34, or optical grade adhesive, having an indexof refraction matching the film, the substrate or some value in between.These layered constructs possess two refractive boundaries: the firstrefractive boundary is disposed at the film exterior or skin exterior 26a, where the film or skin meets the air 32 or adhesive 34; and thesecond refractive boundary is disposed at the substrate exterior 40 a,where the air 32 or adhesive 34 meets the substrate. These refractiveboundaries 26 a and 40 a act as partial mirrors reflecting light at allwavelengths and reducing the overall transmission of the resultingfilter. As can be appreciated, such reduction eliminates the desirablehigh efficiency of the film. This reduction is illustrated by a lightwave passing through the ensemble from left to right in FIG. 1. Notethat as the light wave crosses 26 a and 40 a some portion of the lightis reflected, regardless of the material properties of the intermediatemedium 30.

[0027] Applicants attempted to bond film to substrate without theintroduction of an intermediate adhesive. However, since the filmcontains hundreds of layers, such initial attempts were met withnumerous instances of delaminating portions of the film or destroyingthe optical properties of the film when the film remained intact. Someof these attempts involved thermally bonding or fusing the film to thesubstrate. In contrast to the simple selection of a refractive-matchingadhesive, successful thermal bonding required the optimization of threevariables: the melt flow of the polymer; the melting temperature rangeof the polymer; and the miscibility of the polymer with the outer filmlayer. The goal of the aforesaid optimization is to thermoform athermoplastic substrate onto a film surface without destroying the film.It is believed that the delamination threshold is a function of totalheat energy delivered to the film in a manner where the heat energypenetrates into the film layers and builds faster than it can belaterally dissipated.

[0028] Thermoplastic polymers are typically graded by molecular weightand initially the selection process begins here. While the meltingtemperature range of a polymer correlates approximately with themolecular weight, the melt flow of the polymer correlates reciprocallywith the molecular weight. The melt flow determines the ease with whichthe polymer can be thermoformed via mold, rollers, extrusion or othermechanical former. In addition, it determines the ease with which thepolymer can be delivered to the film surface. However, a low melt flowwill be present with a high melting temperature range leading to filmdelamination or destruction. It was determined that the meltingtemperature range of the polymer must at least partially overlap themelting temperature range of the outer film layer to which it will befused.

[0029] More importantly, the film is constructed of two or morematerials, which have their melting temperature ranges in numericalproximity to each other, but which are not identical. The manner inwhich thermoforming is performed determines the total heat energy whichis delivered to the film. Applicant's discovered that the total energydelivered to the lower melting temperature range layer determines ifdelamination occurs, and hereby defines such phenomenon as thedelamination threshold. In other words, any successful thermoformingprocess must limit the total heat energy delivered into the stack tobelow the delamination threshold.

[0030] Initial restrictions in polymer selection are as follows: thepolymer is miscible with the outer film layer, the molecular weight ofthe polymer provides a melting temperature range that at least partiallyoverlaps the melting temperature range of the outer film layer.Miscibility likely will require matching the polarity of the polymer tothe film layer. Surprisingly, applicants discovered that at the tail endof the thermoforming process, the polymer can be brought into contactwith the outer film layer causing both materials to jointly form apolydisperse layer while staying below the delamination threshold. Suchpolydisperse layer is characterized by a polymeric mix of the materialsvia polymer melting or polymer interfusing creating a continuoustransition there between without intermediate mediums such as air oradhesive. The ensembles according to the invention replaces thoseintermediate mediums with gradual transitions thereby eliminating anyfinite optical boundaries. As illustrated in FIG. 2, the polydisperselayer 42 provides a smooth and continuous transition from the outer filmlayer, i.e. skin 26, to the polymer substrate 40, thereby eliminatingthe two original refractive boundaries. In several instances, theoptical transmission of the film was not lowered by the addition of thepolymer into the optical path, even though the Beer-Lambert Law predictsthat the combined transmission of two filters will be the product oftheir individual transmissions.

Injection Molding Example

[0031] Polycarbonate was maintained in the barrel at 545 to 550 degreesF., well above the film's delamination threshold. The polycarbonate wasinjected into the molding cavity into which the film was previouslyinsert against a 170 degree F. mold surface. As the polycarbonate wasinjected into the cavity it experienced a temperature drop. The portionof the mold adjacent the gate receives the greatest amount of total heatenergy and caused delamination of the film. However, portions of themold spaced from the gate had the polycarbonate tenaciously fused orbonded to the film, and not at all prone to spontaneous or inducedexfoliation. Such bonding strengths were confirmed using standardizedmechanical tests.

[0032] In one test polycarbonate having an average transmission acrossthe visible spectrum of 85% was injected and fused to a near infraredabsorbing film having an average transmission across the visiblespectrum of 79%. When these filters are layered, the Beer-Lambert lawpredicts that transmissions across the visible spectrum will experiencean 85% reduction followed by a 79% reduction, wherein the totaltransmission is calculated as the product of the individual filterstransmissions, i.e. 85%×79%=67%. The filter formed according to theinvention had a 79% average transmission across the visible spectrum.

[0033] In another test polycarbonate having an average transmission of85% across the visible spectrum was injected and fused to an ultravioletabsorbing film having an average transmission of 85% across the visiblespectrum. When these filters are layered, the Beer-Lambert law predictsthat transmission will experience an 85% reduction followed by another85% reduction, wherein the total transmission is calculated as theproduct of the individual filters transmissions, i.e. 85%×85%=72%. Thefilter formed according to the invention had an 82% average transmissionacross the visible spectrum.

[0034] The transmission values are summarized in the following Table1—Transmission Values (in per cent, %) at 50 nm Increments. In Table 1,column 50 lists the wavelengths across the visible spectrum at which thetransmission values were obtained. Note that all values in the columns52 through 62 are percent (%) transmission values. Column 52 indicatesthe transmissions at the listed wavelengths, and at the bottom of thecolumn, average transmission of a Bayer brand water white (clear)polycarbonate substrate. Column 54 indicates the transmissions, andaverage transmission, of a 3M brand organic, optical, dielectric film.Column 56 indicates the Beer-Lambert expected transmission of layeringthe substrate on the film. Column 58 indicates the actual transmissionsof the layered ensemble. This represents the light wave of FIG. 1passing through film 20, skin exterior 26 a, intermediate medium 30which in this instance consisted of air 32, substrate exterior 40 a, andsubstrate 40. Note that the expected, or predicted values in column 56are numerically consistent with the actual transmission values in column58.

[0035] Column 60 indicates the transmissions of the polycarbonate fusedto the film, according to the invention. This represents the light waveof FIG. 2 passing through film 20, polydisperse layer 42, and substrate40, without encountering refractive boundaries between the skin 26 andthe substrate 40. By fusing the substrate to the film, according toinvention, without delaminating the film, the transmission losspredicted by the Beer-Lambert Law is unexpectedly eliminated, resultingin an average 10% increase in transmission. TABLE 1 Transmission Values(in percent, %) at 50 nm Increments PC * Film PC-Film PC-Film % IncreaseBayer Water B-L Law Layered Molded from white (clear) Expected ActualActual Layered to nm PC Film Transmission Transmission TransmissionMolded 450 84.5 83.9 70.9 70.8 81.6 15.3 500 86.5 87.3 75.5 75.5 82.49.1 550 87.4 87.6 76.6 76.5 84.2 10.1 600 87.6 89.8 78.7 78.7 85.8 9.0650 88.3 88.5 78.1 78.2 85.5 9.3 700 89.5 87.7 78.5 78.5 86.1 9.7 75090.1 87.9 79.2 79.2 86.9 9.7 Avg. Avg. Avg. Avg. Avg. Avg. Trans. Trans.Trans. Trans. Trans. Increase 87.7 87.5 76.8 76.8 84.6 10.3 (50) (52)(54) (56) (58) (60) (62)

[0036] At the same time that the transmission values were calculated,applicants obtained the photopic and scotopic transmission values forthe polycarbonate, the film, the layered ensemble and the fusedensemble. The photopic and scotopic transmissions are equal to theindividual transmissions (T) integrated over all visible wavelengths (λ)from 380 nm to 780 nm weighted by the Photopic or Scotopic SensitivityFunction values for that wavelength, namely P_(λ) or S_(λ) furtherweighted by the reference Illuminant curve. Typically illuminant curve C(I_(C)) is employed as the spectrum which best simulates daylight overthe visible range. Thus, the photopic or scotopic value is an integralof three products T, P and I_(C), over the integral of two products, Pand I_(C). The denominator is unity since the P and I_(C) curvesrepresent the normalized 100% reception of available light. The formulafor calculating the photopic is as follows:${\% \quad {Photopic}} = \frac{\int_{{nm} = 380}^{{nm} = 780}{{T_{\lambda} \cdot P_{\lambda} \cdot I_{C,\lambda}}{\lambda}}}{\int_{{nm} = 380}^{{nm} = 780}{{P_{\lambda} \cdot I_{C,\lambda}}{\lambda}}}$

[0037] The water white polycarbonate of column 52 had a % P of 87.3 anda % S of 86.7. The film of column 54 had a % P of 86.7 and a % S of86.2. The layered ensemble had a % P of 75.7 and a % S of 74.4. Theselowered % P and S values correlate to the transmission loss reflected inthe expected and actual wavelength-based transmission data of columns 56and 58. Consistent with the unexpectedly high transmission values ofcolumn 60, the molded ensemble according to the invention, had a % P of84.9 and a % S of 82.3. These values represent a 12.2 and a 10.6%increase, respectively, over the layered values.

Laser Welding Example

[0038] A radiation absorbing material was coated onto a polycarbonateflat and a film was placed over the coated surface. The ensemble wasirradiated through the polycarbonate with a 940 nm diode laser operatingat 50 watts with a 4 mm wide beam. The beam rastered the ensemble at{fraction (1/32)} inch spacing between parallel runs. In areas where thebeam overlapped a previous run, the total energy delivered to the filmexceeded the delamination threshold resulting in localized destructionof the film's optical properties. In other areas the film was fixedlylaminated to the polycarbonate while retaining its optical properties.It is anticipated that a more uniform irradiation than provided byrastering would provide even better results.

Add-On Filter Examples

[0039]FIGS. 3A, 3B, 3C and 3D all show a plastic and film ensembleformed according to the invention and illustrated in FIG. 2. Thecompleted plastic and film ensemble is referred to here as plastic/film50 and includes the plastic side 50 a and the film side 50 b. Theadditional layers, coatings and optical filters shown here are exemplaryof optical devices that could be combined with the plastic and filmensemble according to the invention. The drawings illustrate certaintypes and configurations, although it should be understood thatadditional types and configurations may also be utilized, as will bedescribed more completely below.

[0040]FIG. 3A shows a hardcoat or hardcoat layer 60 placed over the filmside 50 b of the plastic/film 50. Hardcoat 60 provides a layer ofprotection for the film, for example, scratch resistance. Hardcoat 60also provides a surface to receive an anti-reflection layer or coating62. If hardcoat 60 is applied via a dip coating process, then plasticside 50 a will be provided with a hardcoat layer also. Ananti-reflection layer may also be provided on the hardcoat layer onplastic side 50 a of plastic/film 50. Throughout the remainder of thisspecification, an anti-reflection layer means a sputtered metal halidelayer, a sputtered metal calcide layer, a rugate, a dielectric stack, orcombinations thereof. The metal halide layers include metals combinedwith Fluorine, Chlorine, Bromine or Iodine. Metal calcide layers includemetals combined with Oxygen, Sulfur, Selenium or Tellurium. Rugatesinclude metallic and other monolithic constructs having varying indicesof refraction throughout. Dielectric stacks include metallic and otherlayered constructs having varying indices of refraction from layer tolayer.

[0041]FIG. 3B shows a hologram 64 disposed on the film side ofplastic/film 50. A protective optical cap 68 is disposed on hologram 64.An anti-reflection coating may be provided on plastic side 50 a and/orcap exterior 68 a.

[0042]FIG. 3C shows a metal oxide dielectric stack 66 sputtered onto theinterior of cap 68. In the event that optically curved surfaces arepresent, it is preferable that stack 66 be sputtered onto a concavesurface, as shown. The interference spectrum of metal oxide stack 66 isintended to complement the interference spectrum of the organicdielectric film incorporated into plastic/film 50. An anti-reflectioncoating may be provided on plastic side 50 a and/or cap exterior 68 a.

[0043]FIG. 3D shows a hologram 64 disposed on the film side ofplastic/film 50. There is also shown a metal oxide dielectric stack 66sputtered onto the interior of cap 68. An anti-reflection coating may beprovided on plastic side 50 a and/or cap exterior 68 a.

What is claimed is:
 1. An optical filter a thermoplastic substratesupporting an organic, dielectric, optical film without substantiallyreducing the net integrated transmission properties of the film along anoptical path comprising: a dielectric film including (i) repeatingoptical layers of at least two polymers having different refractiveindexes from each other, (ii) an exterior surface material, (iii) adelamination threshold based on total thermal energy delivered to thefilm; a thermoplastic substrate which is miscible with the exteriorsurface material of the film and which is disposed in the same opticalpath as said dielectric film; and a polydisperse region comprising amixture of said exterior surface material and said thermoplasticsubstrate located in the optical path between said film and saidsubstrate, wherein said polydisperse region providing improved opticaltransmission along the optical path in the transition from said film tosaid thermoplastic substrate.
 2. The filter of claim 1, wherein saidexterior surface material has a first melting temperature range and saidthermoplastic substrate has a second melting temperature range thatoverlaps said first melting temperature range.
 3. The filter of claim 1,wherein said thermoplastic substrate includes an organic absorber dye.4. The filter of claim 3, additionally including a hologram disposedthereon.
 5. The filter of claim 3, wherein said organic absorber dye isselected from the group consisting of a UV absorbing dye, a visiblelight absorbing dye, a cosmetic dye, a laser absorbing dye, a nearinfrared absorbing dye, an infrared absorbing dye and combinationsthereof.
 6. The filter of claim 5, additionally including a hologramdisposed thereon.
 7. The filter of claim 1, additionally including ahologram disposed thereon.
 8. The filter of claim 1, further comprisingan anti-reflection coating applied to the thermoplastic material.
 9. Thefilter of claim 8, wherein the anti-reflection coating is selected fromthe group consisting of a metal halide, a metal calcide, a rugate, adielectric stack and combinations thereof.
 10. The filter of claim 8,wherein the anti-reflection coating includes a metal.
 11. The filter ofclaim 1, further comprising a hardcoat layer applied to the organic,dielectric optical film.
 12. The filter of claim 11, additionallycomprising an anti-reflection coating applied to the hardcoat layer. 13.The filter of claim 12, wherein the anti-reflection coating is selectedfrom the group consisting of a metal halide, a metal calcide, a rugate,a dielectric stack and combinations thereof.
 14. The filter of claim 12,wherein the anti-reflection coating includes a metal.
 15. The filter ofclaim 1, further comprising a hardcoat layer applied to thethermoplastic material.
 16. The filter of claim 15, additionallycomprising an anti-reflection coating applied to the hardcoat layer. 17.The filter of claim 16, wherein the anti-reflection coating is selectedfrom the group consisting of a metal halide, a metal calcide, a rugate,a dielectric stack and combinations thereof.
 18. The filter of claim 16,wherein the anti-reflection coating includes a metal.
 19. The filter ofclaim 1, further comprising a hologram applied to the organic,dielectric optical film.
 20. The filter of claim 19, additionallycomprising a protective optical cap applied to the hologram.
 21. Thefilter of claim 20, additionally comprising an anti-reflection coatingapplied to the cap.
 22. The filter of claim 21, wherein theanti-reflection coating is selected from the group consisting of a metalhalide, a metal calcide, a rugate, a dielectric stack and combinationsthereof.
 23. The filter of claim 21, wherein the anti-reflection coatingincludes a metal.
 24. The filter of claim 20, comprising a metaldielectric stack disposed between the hologram and the cap.
 25. Thefilter of claim 24, wherein the cap includes a concave side with themetal dielectric stack sputtered onto the concave side.
 26. The filterof claim 24, additionally comprising an anti-reflection coating appliedto the cap.
 27. The filter of claim 26, wherein the anti-reflectioncoating is selected from the group consisting of a metal halide, a metalcalcide, a rugate, a dielectric stack and combinations thereof.
 28. Thefilter of claim 26, wherein the anti-reflection coating includes ametal.
 29. The filter of claim 1, further comprising a protectiveoptical cap with a metal dielectric layer disposed between the cap andthe organic, dielectric optical film.
 30. The filter of claim 29,wherein the cap includes a concave side with the metal dielectric stacksputtered onto the concave side.
 31. The filter of claim 29,additionally comprising an anti-reflection coating applied to the cap.32. The filter of claim 31, wherein the anti-reflection coating isselected from the group consisting of a metal halide, a metal calcide, arugate, a dielectric stack and combinations thereof.
 33. The filter ofclaim 31, wherein the anti-reflection coating includes a metal.